Anna Mrzljak, Tatjana Vilibic-Cavlek,

Anna Mrzljak, Department of Medicine, Merkur University Hospital, School of Medicine, University of Zagreb, Zagreb 10000, Croatia

Tatjana Vilibic-Cavlek, Department of Virology, Croatian Institute of Public Health; School of Medicine, University of Zagreb, Zagreb 10000, Croatia

Abstract Torque teno virus (TTV) has been proposed as a surrogate biomarker for immune monitoring in different patient cohorts. Historically, TTV has been associated with different liver diseases such as post-transfusion hepatitis, hepatitis B, and hepatitis C, but the virus's pathogenicity is controversial. TTV is a ubiquitous DNA virus, highly prevalent and mostly indolent in the general population. Thus, TTV viral load is more relevant than prevalence to understand TTV infection. In the context of liver transplantation, TTV viral load is modulated by the immune, viral, and inflammatory status. After liver transplantation, the TTV viral load positively correlates with the intensity of immunosuppression (IS), and low TTV viral burden is a predictor of acute rejection episodes, making it an attractive marker for the efficacy of IS. However, the TTV role as a single or a panel biomarker needs to be evaluated in further independent prospective trails.

Key Words: Torque teno virus; Solid-organ transplantation; Biomarker; Liver disease; Liver transplant; Immune system

INTRODUCTION

The presence of torque teno virus (TTV) DNA has been proposed as a novel and attractive surrogate biomarker for the efficacy of immunosuppression in different patient cohorts[1-3]. In solid-organ transplant recipients, immunosuppressive therapy is aimed to prevent rejection and increase organ and patient survival. Usually, a combination of drugs with different action mechanisms is used to control the immune system and prevent/treat the rejection[4,5]. However, the immune monitoring strategies are still based on rough surrogates such as the immunosuppressive drug levels, liver function tests, and biopsies. Other currently available tools are still suboptimal or impractical for the assessment of personalized immune system control[6-8]. In an attempt to optimize the immune system's control, a search for an optimal monitoring tool (e.g., a biomarker) is an ongoing challenge.

TTV

TTV is a non-enveloped, circular single-strand deoxyribonucleic acid (DNA) virus, first identified in Japanese patients with acute post-transfusion hepatitis in 1997[9]. TTV is a member of the Anellovirus family, together with two additional viruses, torque teno mini virus and torque teno midi virus, thus named because of their smaller genomes[10]. Its biological significance is still unknown and evolving. TTV has a high genetic diversity with five genogroups and 29 genotypes identified so far[11]. TTV is ubiquitous, present in water, air, soil, and different human samples[12,13]. The virus's replication has been demonstrated in hematopoietic cells, mononuclear cells and granulocytes, lymphocytes, hepatocytes, and lungs[14-19], reaching far beyond the initially assumed viral hepatotropism. There is no generally standardized diagnostic algorithm for TTV. Polymerase chain reaction (PCR) methods that target TTV can be distinguished as universal, which amplifies most, if not all, the human TTVs, and species-specific, which permits grouping of the virus in one of the 29 TTV genotypes. The diagnosis is focused on the possible pathologic consequence of TTV infection and is performed to measure the kinetics of TTV viremia in selected populations, such as patients treated with immunosuppressive therapy[12].

TTV AND LIVER DISEASES

The first reports on TTV showed low prevalence rates in the general population and patients with liver diseases, most likely due to the use of inappropriate PCR primers[20]. More recent reports demonstrate significantly higher prevalence rates in various liver patients: 77% hepatitis C virus (HCV), 77.7 % hepatitis A virus, 87.6% hepatitis E virus(HEV) and 92% non-A-E hepatitis patients[21]. Historically TTV, has been associated with different liver diseases from post-transfusion hepatitis, HCV, and hepatitis B virus (HBV); however, the pathogenicity of the virus is controversial[13]. The fast-growing evidence shows that the virus infects a great majority of people without causing overt disease. More recent epidemiological studies showed that TTV viremia prevalence rates are over 80%-90% in some populations[22-25], with higher viral load in immunosuppressed patients compared to a healthy population[26]. In addition, the results of one Italian study suggested TTV's role in immune senescence and the prediction of all-cause mortality risk in the elderly. Three-year survival differed significantly by TTV load in a cohort of 379 elderly subjects. The proportion of patients that died after 3 years was estimated to be 21.9% for patients with TTV DNA copies ≥ 4.0 log and 5.4% for patients with TTV copies < 4.0 log. These results indicated that TTV may represent an additional virus that establishes latency after primary infection and reactivates in aging when the immune system is compromised[27].

TTV AND LIVER TRANSPLANTATION

Regardless of the high prevalence and mostly indolent role in the general population, the TTV role in immunocompromised populations needs to be further elucidated. Given the high global prevalence, TTV viral load is more relevant than the prevalence itself to understand the TTV infection[28]. In patients with compromised immune response, TTV viral load increases as the replication of the virus is inversely correlated with the number and function of T lymphocytes[26,29-31]. A substantial body of evidence supports that TTV is more an associated co-factor, but not a major pathogen itself, in the development of post-transplant outcomes. In immunocompromised patients, the low TTV viral burden has been associated with the development of acute rejection episodes in populations after different organ transplantations[32-34]. In addition, higher TTV levels, isolated from the post-transplant lymphoproliferative disease (PTLD) tissues, are shown to predict independently predict death within 5 years of PTLD diagnosis[35]. Studies show that TTV viral load is modulated by immune, viral, and inflammatory status after liver transplantation (LT). Studies evaluating TTV viral load in pediatric[28]and adult LT[3,26,30,36-39]provided evidence that in the early post-LT period, the viral load is higher than before the transplant. Accordingly, the TTV viral load positively correlates with the intensity of immunosuppression[3,26,37]. It progressively increases and peaks around 3 mo post-transplant[3,26,30,37]. After that, the viral load declines, reflecting the progressive reduction of immunosuppressive drugs, to reach a baseline level, on average, after the 1styear of transplant[3]. The viral load is lower in patients with post-LT chronic hepatitis and HEV immunoglobulin M/immunoglobulin G positive patients[28], possibly because the liver is one of the sites of TTV replication. The TTV viral load, however, is not associated with the level of liver enzymes[28]. The pre-transplant TTV status inversely correlates with the acute cellular rejection (ACR) episodes, suggesting that higher immunocompetence in TTV negative patients before the transplant could be responsible for the higher incidence of ACR within 1 year post-LT[38]. Moreover, as confirmed in other transplant populations, lower TTV viral load is associated with the ACR in LT recipients. TTV DNA shows high sensitivity and negative predictive value in the diagnosis of ACR and therefore could be regarded as a non-invasive tool to rule out moderate ACR episodes[3]. Besides, TTV viral loads are associated with the recipient cytomegalovirus (CMV) status; lower levels are present in CMV negative patients[3,30], and early TTV viral load (0-10 d post-LT) is a predictor of CMV reactivation within first 4 mo post-LT[30]. In the context of HBV reactivation in immunocompromised patients including LT recipients, TTV viral load in addition to HBV viral load and HBV genotype are not associated with the development of acute liver/graft failure[40]. Multiple genogroups are frequently found in a single individual infected with TTV. Their distribution differs before and after transplantation, yet it does not affect LT outcomes[28]. Major key points of the LT studies are presented in Table 1.

CONCLUSION

Sophisticated and non-invasive tools to define and/or predict properly the immunerelated events in the post-transplant period are still lacking. The currently available instruments are based on the occurrence of robust clinical events such as rejection or infection episodes. The development and implementation of non-invasive and reliable biomarkers to personalize the immune system's control after transplant remain a challenge. In a search for such a biomarker, collaborative effort over the past decade has brought TTV to the frontline of the medical literature as a promising marker of immune status. The TTV association with the immune status in the immunocompromised transplant population is indisputable. However, we are still looking to understand the impact and the mechanisms behind this interplay. The TTV role as a single or a panel biomarker needs to be evaluated in further independent prospective trials.

Table 1 Torque teno virus in the context of liver transplantation: Major key points